Friday, November 23, 2012

The Bunsen Burner, Spectroscopy, and Geysers

Robert Bunsen (1811-1899)

Robert Bunsen deserves to be remembered for more than his "invention" of the Bunsen burner. He was a brilliant chemist, and did work in spectroscopy, blast furnaces, batteries, and even geology. One of the things that I like about writing this blog is seeing just how many pies these scientists managed to get their fingers in. They were curious, and made discoveries that, even if tangential to their regular work, are in many cases still remembered. See my last post on Bohr for one example. Probably Bunsen's most notable research out of the field of chemistry was his foray into geysers. In the 1840s, on a trip to Iceland to study the recently erupted volcano Mount Hekla. But he also became curious about the geyser there and did some measurements. He came up with a theory about how geysers work, and did a demonstration with a model geyser to show that his theory worked. That would have been a fun demonstration to see!

His early research was in organic chemistry, and he studied arsenic compounds and arsenic poisoning. He showed that iron oxide hydrate could be used as an antidote for arsenic poisoning, and also did extensive research into cacodyl compounds. While still a young chemist, he nearly died of arsenic poisoning and lost the use of one eye from an explosion of one of his arsenic-containing compounds. I have not found anyone to say why he discontinued his studies of cacodyl compounds, but I think it may have had to do with their obviously dangerous effects. The results of his work helped Edward Frankland and Friedrich Kekulé in their studies of chemical valency. Bunsen also studied blast furnaces, which were of great importance in the 1830s due to the huge amounts of iron being produced. He showed that over half of the fuel was lost, and worked with Lyon Playfair to improve the furnaces to be more efficient and to catch potentially useful byproducts. This work resulted in his only book, Gasometry: Comprising the Leading Physical and Chemical Properties of Gases.

When Robert Bunsen became a professor at the University of Heidelberg in 1852, he took charge of a new laboratory building. The building was equipped with gas, and during construction, Bunsen made suggestions to the building's mechanic, Peter Desaga, regarding the burners to be used. There had been previous burners used, including one by Michael Faraday, but his was an improvement on these and enabled the flame to be hot, sootless, and non-luminous. A biographer wrote at his death that "The Bunsen burner is now in use everywhere from the kitchen to the research laboratory." (Crew, p. 302) Not sure how it was used in the kitchen, but there you have it.

Bunsen and Kirchhoff's spectrometer

Though I could also write about his carbon-zinc battery that was much cheaper to make and longer lasting than the previous platinum covered plates, or his invention of the ice-calorimeter and the vapor calorimeter, I do want to talk about his spectroscopic work, which is perhaps most important, and led to the discovery of two new elements. Bunsen's work with spectroscopy was done in collaboration with Gustav Kirchhoff (Kirchhoff's Law, anyone?), whom he met in 1851. He had already been interested in light, such as the improvement of the gas burner and showing that an electric current could create light. Kirchhoff joined Bunsen at Heidelberg and they formed, so it seems, a great team. Bunsen's work with electrochemistry and batteries gave him the ability to separate metals, and the non-luminous burner that he had improved meant that he could use flame tests to see the different colors that metals gave off. Kirchhoff suggested that the colors of different metals that had similar colors might be able to be distinguished by looking at the spectra with a prism. They found that these spectra were unique to different elements. When they noted a new spectral blue line, Bunsen hypothesized that it was a new element and went on to distill 40 tonnes of water to isolate 50 grams of a chloro-platinic coumpound, from which he identified this new element, which he called cesium, Latin for deep blue, from the blue line in its spectrum. In 1861 he announced the discovery of rubidium, and thereafter others used his spectroscopic methods to discover and isolate thallium, indium, germanium, gallium, and scandium.

If you are not familiar with the principle of a flame test, or even if you are because it is always cool, the following video is a nice demonstration of the different colors that different metals are, and shows the spectral lines too.

I will conclude with a sad reminder to back up your data. Bunsen also studied the spectra of rare earth metals, and had just finished a large manuscript on the subject. He left the manuscript on a table near a glass of water, and when he came back, he found the manuscript burnt. It took him two years to replicate the data and apparatuses. So the equivalent of hard drive crashes are nothing new.

Henry Roscoe, "Bunsen Memorial Lecture", Journal of the Chemical Society, Transactions 77, 513-554. DOI: 10.1039/CT9007700513. This is the best biography of Bunsen that I found, and it was written by one of his friends and collaborators.